1. Field of the Invention
The present invention relates in general to the field of signal processing, and more particularly to a method and system for adapting a reference voltage.
2. Description of the Related Art
As the value and use of information continues to increase, individuals and businesses seek additional ways to process and store information. One option available to users is information handling systems. An information handling system generally processes, compiles, stores, and/or communicates information or data for business, personal, or other purposes thereby allowing users to take advantage of the value of the information. Because technology and information handling needs and requirements vary between different users or applications, information handling systems may also vary regarding what information is handled, how the information is handled, how much information is processed, stored, or communicated, and how quickly and efficiently the information may be processed, stored, or communicated. The variations in information handling systems allow for information handling systems to be general or configured for a specific user or specific use such as financial transaction processing, airline reservations, enterprise data storage, or global communications. In addition, information handling systems may include a variety of hardware and software components that may be configured to process, store, and communicate information and may include one or more computer systems, data storage systems, and networking systems.
Information handling systems typically include several integrated circuits installed in a motherboard that communicate data over high speed buses. High speed buses have increasingly relied on input comparator methods to determine whether a signal on a bus is high or low. Typically, each integrated circuit that uses the input comparator method receives a reference voltage generated by a circuit on the motherboard and compares input signals with the reference voltage to determine if the input signal is high or low. The reference voltage value is typically centered between the designed high and low swings of the input voltage to provide as good as possible of a margin to avoid the introduction of errors due to signal noise. For instance, FIG. 1A depicts a resistor voltage divider network circuit 2 used on a motherboard to generate a reference voltage 3 for signals sent over a bus by a GTL driver circuit 4. GTL driver circuits are typically implemented as open drain devices that pull the bus signal to a low state. In the example of FIG. 1, a termination voltage 5 of 1.5 volts and a termination resistor 6 of 50 ohms interfaced with a GTL driver 7 of 25 ohms will result in voltage swings of between 1.5 and 0.5 volts, making 1.0 volts the best theoretical reference voltage with a 0.5 volt margin from each of the high and low voltage swings, as is depicted by FIG. 1B. Resistor divider voltage circuit 2 establishes the reference voltage between two resistors 8 that divide the termination voltage and ground.
One difficulty with the use of a resistor divider circuit is that it generally requires a best guess of a value for the reference voltage, usually a value that is centered between the designed high and low swings of the bus signal input voltage. Once a design value for the reference voltage is chosen, the value typically remains static as a fixed function of the resistor voltage divider network circuit. However, bus input signals typically include a noise component that results in voltage swings that differ from designed voltage swings. For instance, voltage swing characteristics vary depending on the switching patterns generated by the output drivers that generate the bus signal. Voltage swing characteristics are affected by overshoot, ringback, inter symbol interference (ISI), asymmetric NMOS/PMOS driver strength, motherboard impedance and other process variations. These factors typically shift high and low voltage swings from designed values causing some transitions to experience reduced margins when the reference voltage is no longer located between high and low voltage values.